Electric Charge
Electric Charge
It is very difficult, if not impossible, to understand fully what electric charge, q, is. For SAT II Physics, you need only remember the old phrase: opposites attract. Protons carry a positive charge and electrons carry a negative charge, so you can just remember these three simple rules:
  • Two positive charges will repel one another.
  • Two negative charges will repel one another.
  • A positive charge and a negative charge will attract one another.
The amount of positive charge in a proton is equal to the amount of negative charge in an electron, so an atom with an equal number of protons and electrons is electrically neutral, since the positive and negative charges balance out. Our focus will be on those cases when electrons are liberated from their atoms so that the atom is left with a net positive charge and the electron carries a net negative charge somewhere else.
Conservation of Charge
The SI unit of charge is the coulomb (C). The smallest unit of charge, e—the charge carried by a proton or an electron—is approximately C. The conservation of charge—a hypothesis first put forward by Benjamin Franklin—tells us that charge can be neither created nor destroyed. The conservation of charge is much like the conservation of energy: the net charge in the universe is a constant, but charge, like energy, can be transferred from one place to another, so that a given system experiences a net gain or loss of charge. Two common examples of charge being transferred from one place to another are:
  1. Rubbing a rubber rod with a piece of wool: The rod will pull the electrons off the wool, so that the rubber rod will end up with a net negative charge and the wool will have a net positive charge. You’ve probably experienced the “shocking” effects of rubbing rubber-soled shoes on a wool carpet.
  2. Rubbing a glass rod with a piece of silk: The silk will pull the electrons off the glass, so that the glass rod will end up with a net positive charge and the silk will have a net negative charge.
Remember, net charge is always conserved: the positive charge of the wool or glass rod will balance out the negative charge of the rubber rod or silk.
The Electroscope
The electroscope is a device commonly used—and sometimes included on SAT II Physics—to demonstrate how electric charge works. It consists of a metal bulb connected to a rod, which in turn is connected to two thin leaves of metal contained within an evacuated glass chamber. When a negatively charged object is brought close to the metal bulb, the electrons in the bulb are repelled by the charge in the object and move down the rod to the two thin leaves. As a result, the bulb at the top takes on a positive charge and the two leaves take on a negative charge. The two metal leaves then push apart, as they are both negatively charged, and repel one another.
When a positively charged object approaches the metal bulb, the exact opposite happens, but with the same result. Electrons are drawn up toward the bulb, so that the bulb takes on a negative charge and the metal leaves have a positive charge. Because both leaves still have the same charge, they will still push apart.
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